Octave Voyetra Eight, Руководство по техническому обслуживанию аппаратной части, Страница: 18 / 33

The VCF F
c
cutoff is determined by the output of the control summing
amp comprised of the four opamps in the LM324 U6 and associated
resistor networks. The control voltages include:
•
Modulation control
o
Determined by the modulation system and demultiplexed
by CD4052 U1.
•
ADSR 1 control
o
Determined by the CPU and demultiplexed by CD4051 U4
o
Note that the ADSR signals are generated unconventionally
with respect to the standard Demultiplexing scheme (see
DIGIMOD theory)
•
Initial Fc setting
o
Determined by trimmer calibration
•
Keyboard Control and panel Fc setting
o
Determined by the CPU and demultiplexed by CD4051 U3
The sum of these control voltages determines the VCF cutoff while the
adjustable divider network feeding pin 13 on the SSM2044 determines
the filter tracking (Volts/Octave). Increases in control voltage at the
summing amp input produce decreases in control voltage at the divider
network since the summer is an inverting network. The trimmer labeled
“V/Oct” is set so that a change of one volt at the summer input produces
a one octave change in filter frequency. The control voltage is scaled
down to less than +/- 100mV at the SSM2044 input pin 13 to obtain the
full sweep range of 20 Hz – 20 kHz.
Filter resonance is determined by the control voltage at pin 2 on the SSM
2044, which in turn is derived from the difference amplifier LM324 U7.
This difference amplifier receives a signal from the CPU (demultiplexed
at CD4051 U3) that determines the initial resonance for all four VCFs on
the card. The modulation signal coming from the CD4052 U1 comes from
the mod system and is the result of the “Q Destination” settings in the
mod banks. This signal is subtracted from the initial Q voltage and is
scaled by the “Q CAL” trimmer that feeds pin 2 of the SSM 2044.
Because of this subtraction, the resonance increases as the modulation
signal in the mod banks decreases, thus an increasing resonance is
obtained with an inverted source.
4.3 Calibrations
The VOLTS/OCTAVE trimmer is used to set the VCF keyboard tracking.
When set properly, one octave change on the keyboard should closely
correspond to a one octave change in F
c
. The “F
c
INITIAL” trimmer is
used to set all VCFs to the same cutoff for the same key down. When set
properly, the VCFs should all produce an F
c
of A440 when the middle A